Field of the Invention
The present invention relates to a vibration type motor using a vibrator that vibrates by application of an AC voltage, and a lens driving apparatus, a lens unit and an image pickup apparatus, using the vibration type motor.
Description of the Related Art
A vibration type motor using ultrasonic vibration of a piezoelectric element has characteristics of compactness, attainment of a high drive force, operativeness in a wide speed range, low vibration and low noise. Because of these characteristics, the vibration type motor is used for linear drive of a focus lens and for rotational drive of a cam cylinder in, for example, a lens barrel of a camera that needs compactness and high-output drive.
As a drive principle of the vibration type motor, there has been known that a vibrator vibrates using expansion and constriction of a piezoelectric element by piezoelectric effect and the vibration is transmitted to a friction member that is brought into pressed contact with the vibrator so as to relatively move the friction member. An apparatus disclosed in Japanese Patent Application Laid-Open No. 2004-304887 is an example of the vibration type motor.
The apparatus disclosed in Japanese Patent Application Laid-Open No. 2004-304887 is a vibration type motor in which a flat plate-shaped vibrator provided with a projection generates a plurality of vibration modes and causes the projection to generate elliptical movement by a combination of the vibrations modes. The projection is in pressed contact with the friction member, and when the projection is caused to generate the elliptical movement, the projection feeds out the fiction member while repeatedly contacting with and separating from the friction member. Accordingly, the friction member is driven relatively to the vibrator.
On this occasion, if a resonance frequency of the friction member overlaps with a drive frequency band for vibrating the vibrator, resonance of the friction member is induced to cause unnecessary vibration, thereby decreasing drive force and generating noise. To address the problems, Japanese Patent Application Laid-Open No. H9-215348 has proposed to design a structure in which a resonance frequency of the friction member is set within a predetermined range in such a manner that a drive frequency band of the vibrator and the resonance frequency of the friction member do not overlap with each other.
Even in the case where the resonance frequency of the friction member designed to be set within the predetermined range, due to slight variations in dimension of components and in fixing method, the resonance frequency of the friction member is changed. Accordingly, even in the case where the resonance frequency of the friction member is designed to be set within the predetermined range so as not to overlap with the drive frequency band, it has happened that the resonance frequency of the friction member has overlapped with the drive frequency band for vibrating the vibrator in manufactured products, thereby reducing drive force and generating noise.
Specifically, in a case where the friction member is manufactured by a manufacturing method that is inexpensive in costs such as press forming, it is difficult to form a corner portion of the friction member stably so that the resonance frequency cannot be set within the predetermined range and the driving force is decreased and noise occurs easily.